Membrane pumps are a well known type of pump in which a reciprocating piston is replaced by a membrane so that no liquid or gas, as determined by the use to which the pump is made, leaks out of the pumping chamber during use. Such could occur in a pump having a worn piston. The assignee of the present invention is the owner of U.S. Pat. No. 6,776,591 (hereinafter the “'591 patent”), entitled Membrane Pump Comprising An Inlet Opening That is Controlled By The Membrane; this patent is incorporated herein in its entirety as if set forth in full herein. The '591 patent describes a pump using a membrane along with the advantageous placement of the inlet and outlet ports of the pump such that maximum compression and pump efficiency can occur. The design of the pump taught in the '591 patent provides for an efficient and low cost pump capable of high compression as a result of the advantageous placement of the inlet and outlet ports such that the membrane closes the inlet port of the pump at a position allowing the greatest potential compression and therefore pumping efficiency.
Often times, however, a two stage pump is required to perform the necessary pumping operation, providing extra pumping power not otherwise possible with the single stage pump. For example one pump is designed to draw a fluid or gas from one location and the second pump is designed to accept that liquid or gas and pump it to a destination. Typically the two-stage pump is constructed of two single stage pumps joined together in series, and held in a single housing, such that the outlet of the first stage feeds the inlet of the second stage. Often times a camshaft system is aligned such that the pumps operate synchronically such that when the first stage is at a top dead center the second stage is at a bottom dead center position. In prior two-stage pumps, each of the pumps that make the two stages are typically identical pumps having identical equipment. The two stages are therefore really two pumps working in unison. A two stage pumping system is therefore usually twice as expensive to manufacture as a single stage pump and requires substantially more maintenance to maintain each pump as well as the synchronicity of the pumps working together.
It would therefore be an advantage to have a two stage pump that requires fewer parts and therefore less cost to manufacture, as well as requiring concomitantly less maintenance.